Heretofore, the fixed specimen technique has been employed to obtain transmission type images. The specimen is irradiated with ultrasonic energy, and a single acoustical transducer is systematically moved about the specimen in order to determine the amplitude and phase of the pulses transmitted through the specimen as a function of position. (See Applied Physics Letters 11, page 19, 1967, "Simulated Reference in a Coarsely Sampled Acoustical Hologram" by A. F. Metherell and H. M. El-Sum.) Alternatively, both the source transducer and the receiving transducer may be fixed and the specimen rotated to obtain transmission data at various angles through the object. G. N. Hounsfield describes a moving detector spaced from a cw X-ray source having a fixed specimen therebetween (see "Computerized Transverse Axial Scanning-Tomography" in the British Journal of Radiology, 46, pages 1016-1047, Dec. 1973). Transmission data from each detector are reconstructed into an image by a computer. These prior art techniques require close tolerance moving parts to provide the mechanical rotating or scanning.
Reflective imaging techniques have also been employed in the prior art. The reflective technique relies on changes of transmission impedance, which generate a series of reflected pulses spaced according to the depth of each reflecting layer within the specimen. These range pulses respond to impedance changes and do not yield information about the absorption coefficient or velocity coefficient within the specimen.